Three-dimensional videos are becoming popular not only as large screen movies but also as TV content and game footage with the growing popularity of household three-dimensional TVs and computer displays. A three-dimensional video gives a viewer illusion as if the video has depth by creating a disparity between respective images presented to the right and left eyes. Unlike the conventional two-dimensional videos, when viewing a three-dimensional video, the right eye and the left eye move along with the depth of a subject. In other words, when the depth is small and the subject appears projected forward, the left and right eyes move in a direction in which the viewer goes cross-eyed, when the depth is great and the subject appears at far distance, a state of the eyes is close to a state in which the eyes look straight ahead. On the other hand, because the position of a display remains fixed, the viewer is required to continuously be focused at the position of the display for clear viewing. Thus, the focal length of the video is fixed to the position of the display. Due to such conflict between the focal length and the eye movement to the virtual depth of the subject, it is conceived that physical symptoms such as fatigue and video-induced motion sickness may appear in the viewer involved with viewing a stereoscopic video. Thus, conventionally, the viewer fatigue is determined, in PTL 1, by measuring the optical characteristics of the viewer's eyes viewing the three-dimensional video.
On the other hand, to present different images to the left and right eyes using one display, it is necessary to use dedicated eyeglasses (Hereinafter, also referred to as “three-dimensional eyeglasses.”) and use shutters to select light directed to the left and right eyes. Switching the shutters in synchronization with right-eye and left-eye screen displays at a sufficiently high frequency allows the viewer to perceive a stereoscopic view, fusing information to the eyes without noticing that the right-eye and left-eye images are being switching.
Meanwhile, during the display of the three-dimensional video, when a viewer sees the display without wearing the dedicated eyeglasses, the viewer not only cannot perceive the stereoscopic view but also sees double imaging. Thus, in PTL 2, to assuredly achieve a stereoscopic view, whether a viewer is wearing the dedicated eyeglasses is sensed in contact or non-contact with the dedicated eyeglasses, and the dedicated eyeglasses transmit a signal indicative of the wearing conditions to a display device which controls video display, such as TV, computers, and game consoles. This controls the switching of the two-dimensional video display and the three-dimensional video display in the display device.